<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="../../js/jquery-1.11.3.js"></script>
</head>
<body>
<input type="button" value="请求">
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<p>asdas</p>
<h1>asdsa</h1>
<div class="js_head">

</div>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>
<p>aa</p>

<script>
    $(function(){
        var flag = 0;
        $(window).scroll(function(){
//            console.log("aa")

            if ($('h1').offset().top<=$(window).scrollTop()){

                if (flag == 0){
                    $.ajax('a.html',{
                        success:function(data){
                            console.log(data);
                            $(".js_head").html(data);
                            flag = 1;
                        }
                    })
                }

            }
        })
    });
</script>


</body>
</html>